This invention relates to a method of forming methacrylamide polymer composites using electro-polymerization techniques. More particularly, this invention relates to the formation of high glass transition temperature polymer coatings onto the surface of conductive filler materials (such as graphite fibers) wherein the polymer is derived from cyclic N-substituted methacrylamide monomers. This invention provides an electropolymerized polymer composite which is processible as a thermoplastic, but undergoes a crosslinking step upon heat curing so as to exhibit thermoset properties such as high strength, high glass transition and high resistance to flow and solvents.
High performance resins with a good long term environmental stability over a wide range of temperature and with damage tolerance are currently in great demand, particularly in aerospace applications. These resins are typically needed in the manufacture of composite materials which require the properties of high strength and low weight. Thermosetting resins such as epoxy systems are the most widely used matrix resins for such advanced composites. Unfortunately, they generally possess insufficient hot/wet properties, toughness and temperature resistance. An exception to this generality is bismaleimide polymers since they are stable at elevated temperature (approximately 200.degree. C.) and have good hot/wet properties. The problem with such systems is that the processing times are relatively long and there is no resin flow after the cure reaction is completed. Greater toughness and impact resistance are also highly desirable.
These problems have heightened the search for alternative thermoplastic materials for use as matrices for advanced composites Advantageous features of thermoplastic matrices include high toughness, easy processibility, long shelf life and potential for high volume processing resulting in low cost per part. However, the difficulty in preparation of prepregs from high viscosity thermoplastic resins and the problem of wetting all the individual fibers in a fiber bundle as well as the problem associated with polymer solubility and solvent removal limit the efficiency of use of thermoplastics in advanced composites.
Electropolymerization has been used in the direct formation of polymers onto electrode surfaces such as graphite fibers. Such polymerizations have generally been from non-aqueous solutions, by ionic or mixed ionic-radical mechanism. However, molecular weights of the electropolymers from these solutions have generally been low.
More recently, aqueous solution electropolymerization techniques have been disclosed to apply thin (less than ten (10) weight % polymer) polymeric coating onto graphite fibers. See Bell et al, Polymer Composites, 8,46 (1987), Subramanian et al, Polymer Engr. Sci., 18,590 (1978). Unfortunately, such thin deposition of thermoplastic polymers onto graphite fibers does not satisfy the need for new thermoplastic composite materials since the thin coatings are incapable of forming the required thick thermoplastic matrix needed in such composites.
An improved electropolymerization method and composite product derived therefrom is disclosed in U.S. patent application Ser. No. 366,933 filed Jun. 16, 1989, assigned to the assignee hereof and fully incorporated herein by reference. In accordance with the technique of U.S. Ser. No. 366,933, electropolymerization in a substantially aqueous solution is used to form thick (e.g. greater than 2 microns or 30 weight %) and thermally stable coatings of thermoplastic materials onto electrically conductive materials (e.g. rods, plates, fibers, film or cloth). In a preferred embodiment, the thick thermoplastic matrix comprises a copolymer of 3-carboxyphenyl maleimide (3-CMI) and styrene. Also disclosed are thermoplastic matrices comprised of glycidyl acrylate/methyl acrylate copolymer as well as certain other thermoplastic polymers and copolymers.
The technique of U.S. Ser. No. 366,933 is particularly well suited for direct preparation of thermoplastic prepregs containing commercially available bundles of graphite fibers. These prepregs are then molded under heat and pressure so as to form a thermoplastic matrix composite with good fiber distribution, uniformity and high temperature resistance.
In view of the favorable results and advantages of the above-discussed electropolymerization technique, there is a perceived need for additional development of electropolymerized thermoplastic polymer composites exhibiting good mechanical and thermal properties.
Experimental work is known with regard to the electropolymerization of acrylamide monomers (see M. Cvetkovskaja, T. Grcev, L. Arsov and G. Petrov, Kem. Ind. 34, 235 (1985) and J. R. MacCallum and D. H. MacKerron, The Electropolymerization of Acrylamide on Carbon Fibres, British Polymer Journal, Vol. 14, March 1982, pp. 14-18). However, the electropolymerized polyacrylamide composites described in the prior literature suffer from certain disadvantages and deficiencies including high water absorption, low temperature resistance, high solubility rates in water and thin polyacrylamide coatings on fillers such as graphite fibers.